The present invention relates to a laser beam machining method and apparatus for irradiating a work with a laser beam, to melt-evaporate the irradiated region of the work at the irradiation spot. Particularly the present invention provides a laser beam machining method and apparatus that can machine the tip portion of an optical fiber, as an example of the work, into a desired form.
Optical fibers, for example, optical fibers mainly composed of quartz glass are used in optical transmission systems and other optical systems, and the tip forms of these optical fibers play an important role irrespective of kinds of fibers such as single mode fibers and multi-mode fibers.
It is desired that the end faces of an optical fiber have an especially smooth surface and an accurate form for minimizing the connection loss in its connection with another optical fiber or an active device. Therefore, it is desired that the method of machining the tip of an optical fiber can achieve accurate machining into a predetermined form at high productivity
Known methods of finely processing the tip of an optical fiber include mechanical methods such as fiber cleaving, chemical methods such as etching and optical methods such as the use of a CO2 laser, etc.
The mechanical method using a fiber cleaver allows the tip of an optical fiber to be simply and sharply cleaved, but has a problem that it cannot process the tip into a semi-spherical, conical, or wedge-like surface, etc.
The chemical method using etching allows the tip of an optical fiber to be formed as desired, but since it is difficult to control the form and takes a long period of time, the method has a problem in view of productivity.
In the case where the conventional general method of using a CO2 laser is used to cut an optical fiber to process it at an end, it can happen that the heat generated during machining causes a form error, and since the spatial distribution of light intensities is Gaussian, there is such a problem that the machined edge becomes blunt.
Examples of these cases are described below.
For example, in the optical fiber cutting methods and apparatuses described in the gazettes of JP02-230205A and JP02-238406A, optical fibers are mechanically cut. These methods allow optical fibers to be cut easily and well, but cannot be used for processing the tips of optical fibers.
EP 0987570 discloses a method of cutting an optical fiber using a pulse CO2 laser. In this method, a circular laser beam with Gaussian-distributed light intensities is merely condensed by a lens for cutting an optical fiber. The method cannot process the tip of the optical fiber into a desired form.
U.S. Pat. No. 5,256,851 discloses a method of melt-evaporating the tip of an optical fiber very little by very little using a pulsed CO2 laser. This method has such problems that it takes a long period of time for predetermined machining.
In the above-mentioned machining method using a pulsed CO2 laser, since the tip of an optical fiber to be differently formed depending on the applicable specifications must be processed into a desired form by repeating micro machining, the laser beam must be finely condensed like a point using a lens, for accurate processing into a desired form.
Therefore, the control of the laser beam irradiation position for adaptation to the form to be obtained at the tip of the optical fiber is troublesome, and expensive equipment is necessary for very highly accurate irradiation position control.
In addition, the spatial distribution of light intensities, i.e., profile of the light condensed by a lens becomes conical with the focus as the vertex, machining becomes difficult with the increase in the depth of the machined portion of the optical fiber. Furthermore, there is such a problem that since a thin V-shaped end face is formed in the section of the machined portion, the gas, fume and heat generated during machining are likely to be retained there, to contaminate or curve the machined surface.
Furthermore, if the pulsed CO2 laser is used, since the tip of an optical fiber is irradiated with a laser beam having high light intensity continuously for a long time, the peripheral portion of the tip portion is also heated to deform the optical fiber, not allowing machining as designed. Moreover, if a pulse laser is used, the thermal effect on the optical fiber can be reduced, but there is another problem that the machining time becomes longer by that.
One of the objects of this invention is to provide a laser beam machining method and apparatus that can machine the tip of a work such as an optical fiber into a desired form highly accurately within a short period of time.
Another object of this invention is to provide a laser beam machining method and apparatus capable of preventing the vibration of the fiber caused by the ablation during laser beam irradiation and preventing the occurrence of facial sagging, that respectively lower the form accuracy at the cut face when the tip of an optical fiber is cut by means of laser beam machining.
To solve the above-mentioned problems, the present invention proposes a laser beam machining method, in which a work is irradiated with a laser beam to melt-evaporate the portion irradiated with the laser beam for machining the work, characterized in that
a mask having a light-transmitting section that is predetermined times as large as the laser beam machining spot corresponding to the form of the portion undergoing melt-evaporation of the work, is disposed between a laser beam source and the work, and
the laser beam transmitted from a laser beam source through a beam-shaping optical system is irradiated to said mask in a range larger than said light-transmitting section, and the real image of the light-transmitting section formed by the transmitted light is reduced to the size of said machining spot by means of a reduced image-forming optical system, to form the reduced image on the work, for machining.
According to this method, since the light-transmitting section that is predetermined times as large as the machining spot corresponding to the form of the portion undergoing melt-evaporation of the work is formed in the mask irradiated with a laser beam, the laser beam irradiated in a range larger than the light-transmitting section passes through the light-transmitting section of the mask, to become a beam with a spot form equal to the form of the light-transmitting section.
This beam passes through a reduced image-forming optical system, and as a result, the real image of the light-transmitting section is reduced to the size of said machining spot, to form the reduced image on the work. The portion of the work in the range of the machining spot is melt-evaporated, and the portion not melt-evaporated remains as a desired form.
In this case, if the laser beam passes through the light-transmitting section formed in the mask, the light intensity at the edge portion of the optical beam becomes high due to light interference. So, the portion undergoing melt-evaporation can be well molten also at the area corresponding to the boundary with the portion undergoing no melt-evaporation, and the thermal effect on the portion undergoing no melt-evaporation is small.
As described above, simply by forming the light-transmitting section of the mask with a large area as desired, the real image of the light-transmitting section can be reduced to the size of said machining spot, to form the reduced image on the work. So, when the work is machined, it is not necessary to control the laser beam irradiation position each time.
Furthermore, since the laser beam is not condensed like a point on a work for irradiation as in the conventional method, but is irradiated as said machining spot corresponding to the form of the portion undergoing melt-evaporation, the portion undergoing melt-evaporation can be melt-evaporated generally as a plane not as a point, and the predetermined machining can be accomplished within a short period of time.
This invention also proposes a laser beam machining method, in which a work is irradiated with a laser beam to melt-evaporate the portion irradiated with the laser beam for machining the work, characterized in that
a mask with a quadrilateral light-transmitting section that is predetermined times as large as a quadrilateral machining spot is disposed between a laser beam source and the work, and
the laser beam transmitted from the laser beam source through a beam-shaping optical system is irradiated to said mask in a range larger than said light-transmitting section, and the real image of the light-transmitting section formed by the transmitted light is reduced to the size of said machining spot by means of a reduced image-forming optical system, to form the reduced image on the work, for machining.
In this case, if the real image-forming position is moved relatively along the laser beam irradiation axis, while the work is machined, the image-forming face can be made to agree with the machined face at each position in the depth direction of the work. So, a well-machined face can be obtained without lowering the machining speed.
Furthermore, if the real image-forming position is reciprocated relatively in the direction perpendicular to the laser beam irradiation axis, while the work is machined, the machining method of this invention can be adapted to a large work.
Moreover, in the case where a work is machined in the direction crossing the axial direction of the work, if the axial direction of the work is held at a setting angle xcex1 expressed by
xcex1=(xcfx80/2)|tanxe2x88x921(d/2f)xe2x88x92xcex2
(where xcex2 is the angle of the face to be machined against the work reference face perpendicular to said axial direction, d is the width of the transmitted light incident from the quadrilateral light-transmitting section of the mask on the reduced image-forming optical system, and f is the focal distance of the reduced image-forming optical system) against the laser beam irradiation axis, the face to be machined of the work can be machined at a desired angle.
If the work is relatively revolved around the axis perpendicular to the laser beam irradiation axis, when machined, a surface of revolution such as a semi-spherical surface, conical surface or paraboloid of revolution can be formed.
If the real image-forming position is relatively moved along the laser beam irradiation axis while the work is machined, or if the real image-forming position is relatively reciprocated in the direction perpendicular to the laser beam irradiation axis, when machined, then the work can be adequately machined, depending on the kind and form of machining, the width, thickness and size of the portion to be machined, etc.
The work can be, for example, an optical fiber, and the tip of the optical fiber can be machined variously. If the optical fiber is, for example, fixed in a glass capillary, when machined, it is possible to prevent the vibration of the fiber caused by the ablation during laser beam irradiation and to prevent the occurrence of facial sagging that respectively lower the form accuracy at the cut face formed by laser beam machining, etc.
The present invention also proposes a laser beam machining apparatus for applying the above-mentioned method, characterized in that a mask is disposed between a laser beam source and a work held by a holding means; a light-transmitting section that is predetermined times as large as the laser beam machining spot corresponding to the form of the portion undergoing melt-evaporation of the work is formed in the mask; a shaping optical system for irradiating said mask with a laser beam in a range larger than said light-transmitting section is disposed on the laser beam source side of the mask; and a reduced image-forming optical system for reducing the real image of the light-transmitting section formed by the laser beam passing through the light-transmitting section of the mask, to the size of said machining spot, and for forming the reduced image on the work, is disposed on the work side.
The laser beam machining apparatus can have a constitution in which the work holding means is provided with a rotating means for rotating the work around its axial direction, or a constitution in which a moving means for moving the real image-forming position along the laser beam irradiation axis relatively to the work is provided, or a constitution in which a moving means for reciprocating the real image-forming position in the direction perpendicular to the laser beam irradiation axis relatively to the work is provided.
It is preferred that the laser beam for machining as described above is a multi-mode beam having a flat beam profile, and the light source can be, for example, a TEA-CO2 laser (Transverse Excited Atmosphere CO2 laser).
If a pulsed laser beam is used as the laser beam, the above-mentioned thermal effect can be further inhibited to improve the machining accuracy.